The present invention relates generally to optical signals and also to electrical circuitry.
It is desirable to measure the received power of an optical signal in an optical communication system. The received power measurement is useful in monitoring and controlling operation of an optical communication link.
One way to accomplish this measurement would be to tap off a portion of the received optical signal and then measure the power level of this tapped portion. However, the necessary fiber taps, splices, and the routing and management of fiber are both cumbersome and expensive. Also, the total received signal level may be quite small, bringing the power level of a tapped signal close to or below the threshold of accurate measurement.
Accordingly, a class of optical power measurement techniques has been developed that take advantage of the photodiode used to recover the modulation on the optical signal. These techniques measure the current through this photodiode and apply a multiplicative factor to estimate the received power.
A typical power measurement circuit of this type involves placing a sensing resistor in series with an avalanche photodiode (APD). The voltage across this resistor is sensed, converted to a proportionate current by a voltage to current converter, and input to a logarithmic amplifier. Measurement inaccuracies result from variations in the sense resistor's resistance over temperature and voltage-to-current converter offset and gain variations due to temperature changes. Resolution at low power levels is also very poor.
Another approach avoids the problems inherent in using a resistor to sense voltage by using a current mirror to measure the current through the APD. Conventional current mirrors, however, are not very accurate over variations in temperature and current level. Precision current mirrors do not operate well at the high voltage bias levels demanded by APDs.
What is needed are improved systems and methods for precisely measuring optical power levels.